The present invention relates generally to optical communication networks and, more particularly, to a control system and method for an optical amplifier.
Telecommunications systems, cable television systems, and data communications networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers comprise thin strands of glass capable of transmitting the signals over long distances with small loss.
Optical networks often employ wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM) to increase transmission capacity. In WDM and DWDM networks, a number of optical channels are carried in each fiber at disparate wavelengths. Network capacity is based on the number of wavelengths, or channels, in each fiber and the data rates of the channels.
To increase the signal strength over long distances, optical communications systems typically include optical amplifiers at or between network nodes. The amplifiers typically include automatic gain control (AGC) to maintain a desired amplification factor (gain) across the amplifier. An optical amplifier may be used for each wavelength or channel transported by a fiber; however, using one amplifier for all wavelengths reduces system costs.
The present invention provides a control system and method for an optical amplifier. In a particular embodiment, pure-electric feedforward and feedback controls are provided for an optical amplifier to provide sub-microsecond response time to fiber cuts and other fast channel changing events.
In accordance with one embodiment of the present invention, a system and method for controlling an optical amplifier includes controlling the optical amplifier pump lasers with electrical feedforward and feedback circuits. In the feedforward portion, based on the measured total input power, a feedforward pump power is determined. The pump power is adjusted based on the determined pump power. In the feedback portion, an output power is measured and gain is determined based on the output power and the measured input power. The measured gain is compared to a desired gain and the pump power is adjusted based on that comparison.
Technical advantages of the invention include providing an improved control system and method for an optical amplifier. In one embodiment, an optical amplifier includes automatic gain control (AGC) with nominal feedforward and compensation feedback control that greatly improves response time down to sub-microsecond speeds for fast channel adding or dropping processes, such as, for example, a fiber cut. In particular, the nominal feedforward control monitors the total input power of signals into the amplifier, and provides a nominal pump current to the pump laser of the amplifier based only on the total input power. Because the pump power is changed immediately after the change in a number of channels of the input signal, no extra energy is stored in the gain medium of the amplifier and no excess population inversion is generated during the transition process; therefore, no gain excursion will be generated. As a result, better protection is provided to downline network components by reducing the probability of a large power spike or power drop. In addition, transmission errors caused by power fluctuations are limited or minimized.
Another technical advantage of the present invention includes providing a pure-electric controlled AGC for an optical amplifier. In one embodiment, the electric controlled AGC includes the nominal feedforward control and a compensation feedback control. The pure-electric AGC provides fast response times without the need for new optical components or extra pump power. As a result, costs of the amplifier and/or amplifier control are limited or minimized.
Still another technical advantage of the present invention includes providing nominal feedforward control for an optical amplifier with aging factor compensation. In particular, pump laser aging will increase the required pump current for an input power, which will affect a predefined nominal value of pump current used for feedforward control. Because the pump power is always monitored in the optical amplifier, the aging factor can be determined automatically by the built-in control unit and an aging factor applied.